Method of cooling pulp suspensions in grinding and refining operations



Aug. 29, 1967 A. J. A. ASPLUND ET AL 3,338,525

METHOD OF COOLING PULP SUSPENSIONS IN GRINDING AND REFINING OPERATIONS Filed Nov. 25, 1964 E Sheets-Sheet 1 INVENTORS ARNE JOHAN ARTHUR ASPLUNli KARL NICOLAUS CEDEROUIST ROLF BERTIL REINHALL g- 29, 1967 A. .1. A. ASPLUND ET Al. 3,338,525

METHOD OF CGOLING PULP SUSPENSIONS IN GRINDING AND REFINING OPERATIONS Filed Nov. 25, 1964 ZSheets-Sheet 2 ARNE JOHAN ARTHUR ASPLUND KARL NICOLAUS CEDERQUIST ROLF BERTIL REINHALL United States Patent Office 3,338,525 -Patented Aug. 29, 1967 3,338,525 METHOD OF COOLING PULP SUSPENSIONS IN GRINDING AND REFINING OPERATIONS Arne Johan Arthur Asplund, Lidingo, Karl N. Ceder- 'j quist, Nasbypark, and Rolf Berti] Reinhall, Lidingo, Sweden, assignors to Defibrator Aktiebolag, Stockholm, Sweden, a corporation of Sweden Filed Nov. 23, 1964, Ser. No. 413,207 Claims priority, application Sweden, Mar. 10, 1964, 2,986/64 4 Claims. (Cl. 241-17) This invention relates to a method of cooling pulp suspensions in grinding and refining operations.

Experiments carried out in both laboratory and factory scale have shown that the'grinding of pulps rich in hemicellulose and lignin in refiners of various types, such as disc refiners, for example, must be conducted below 100 C. if the grinding result shall become satisfying from the viewpoint of paper manufacture. The pulp types contemplated here comprise all kinds of sernichemical, chemomechanical and mechanical pulp made of wood chips which are produced with high yield ranging from 75 to 95 percent and contain a substantial quantity of lignin and'hemicellulose.

When using as starting material in the grinding and refining processes a spruce stock produced with a yield of 94 percent'by'decomposition of wood chips impregnated with 1 percent sulfite, calculated as Na O, and heated under pressure at 175 C. in a steam atmosphere during two" minutes and thereafter defibrated under prevailing pressure and temperature according to the Asplund or Defibrator method, one will observe that the grinding process becomes quite different if the grinding is effected Within the temperature range from 20 to 100 C. or if effected in the temperature range from 100 to 150 C.

1 In the grinding of the stock specified above in a pulp suspension having a concentration of 12 percent and an average temperature of about 60 C. in a refiner of specific type the freeness of the pulp decreased from 720 to 295 CSF within 15 minutes. If the same processing was, however, conducted at an average temperature of 125 C. the freeness value was lowered in 15 minutes to 680 CSF only and after additional 15 minutes to 630 CSF. At temperatures exceeding 100 'C. the pulp proved to encounter much difficulty in becoming. subjected to refining. An inspection under a microscope of the ground pulp shows distinct differences between cold refined and warm refined pulps. Cold refined pulp shows better fibrillation and contains-a-substantially minor quantity of torn-off fibers. -It is known since long time ago that lignin in moist state'begins to soften at a temperature of about 100 C. However, not until quite recently it has been discovered that the hemicellulose also softens and that this transformation sets in at a temperature of 50 to 60. Concern ing the stocks of the kinds in consideration here it appears suspension rises. In many cases this rise may become so high that the temperature range is reached within which the refining of the fibers is impossible to be conducted in a satisfying manner.

In the grinding and refining of highly diluted fiber suspensions having for example a concentration of 6- percent and lower, a cooling is usually not necessary due to the great heat capacity of the pulp suspension which prevents the temperature from rising too much. If in the case in consideration a cooling nevertheless becomes necessary or desirable, such cooling can be effected very simply in an indirect way by means of heat exchanges because said suspensions have a relatively low viscosity and are easy to be pumped. In modern practice, however, it is necessary that the grinding and refining of semi-chemical pulps and mechanical pulps is conducted with the pulps having a high concentration of between about 10 and percent. Under these conditions the grinding result becomes the optimum regarding the properties of the pulps for paper manufacture and the consumption of energy for attaining a predetermined freeness. The tem perature rise is then liable to become considerable and a cooling will become necessary. Due to the high viscosity of the pulp suspension and suspensions of high concentration the indirect cooling method mentioned herein'before cannot be applied without expensive technical arrangements. One main object of the invention is to overcome the difficulties set forth in a novel way by providing a method which renders poscomprises between 35 and 45 material, is in a plastic state at temperature about or immediately above 100 C. and that at a temperature of C. already part of the 'intercellular substance has begun to be converted into an amorphous plastic mass. Itis probable that this plastic state drastically obstructs the fibrillation and prolongs the grinding time required for attaining a desired or predetermined freeness. It has also been established that the temperature in grinding and refining processes must not exceed 100 C. and suitably not surpass C. The best results have been attained with temperatures not exceeding 60 to 70 C.

In the grinding of a fiber suspension practically all the mechanical energy supplied is practically totally converted into heat resulting in'that the temperature of the fiber percent of the total fiber which form part of this specification sible to keep the pulp suspensions during the grinding and refining treatment at temperature not exceeding the critical value set forth above. my

Experimentshave established that a simple and effec tive cooling of pulp suspensions in grinding and refining processes can be brought about by conducting the grinding operation under a negative pressure. It is thus a further object of the invention to provide a method and means permitting the grinding operation to be conducted under negative pressure.

By choosing a suitable difficulties encountered in adjusting the temperature to any desired value between 20 and C. the developed heat is removed as vapor enthalpy and a constant temperature equivalent with the boiling point of the water at the prevailing negative pressure can be maintained for the suspension. Consequently, it is a still further object of the invention to provide a method of grinding and refining pulp suspensions allowing the suspensions during the entire grinding and refining operation' to be kept at any desired more or less constant temperature between 20 and 100 C. How this cooling can be realised in practice depends on the'type of refiners and whether or not continuous grinding shall' be applied. It is thus a still further object of the invention to provide a method of cooling pulp; suspensions in grinding and refining treatment in response to the type employed of refiners and the processing method. Y Further objects and advantages of the invention will become apparent from the following description, considered in connection with the accompanying drawings and of whichz FIG. 1 is a more or less diagrammatical'lateral eleva; tion of a plant constructed according to the invention.

FIG. 2 is a similar lateral elevation of a plant according to a modified embodiment of the invention;

FIG. 3 is a sectional view line III-III of FIG. 2.

The embodiment according to FIG. 1 illustrates a re fining operation carried out in three stepswith a coolingby applying negative pressure to the two last steps thereof. The refining is intended to be effected in a disc refiner negative pressure there areno difficulties in pumping pulpon a larger scale following with a chemo-mechanical spruce stock manufactured in the manner described hereinbefore and with the same yield. The freeness of the defibrated pulp is about 720 CSF and the pulp shall be ground and refined to a freeness value of 150 CSF. In the grinding and refining to a freeness of 150 CSF totally 700 kwh. are consumed per one ton of stock which value when divided to the three steps gives about 230 kWh. per one ton of stock in each step.

Referring to FIG. 1, reference numeral denotes a centricleaner into which the digested pulp ground in a defibrator (not shown) is introduced through a conduit 12. The stock may be withdrawn through a sluice from the defibrator in which superatmospheric pressure exists, accompanying steam and uncondensable gases escaping from the centricleaner 10 through a pipe 14. The pulp is assumed to have a bone-dry content of about 17 percent and a temperature of 100 C. upon having left the defibrator. Cooling water having a temperature of 20 C., for example, is at the sametime supplied to the centricleaner 10 through a pipe 16 in such quantity that a pulp concentration of about 10 percent is obtained in the bin 18 disposed under the centricleaner. The suspension there has a temperature of about 40 C. Disposed in the bin 18 may be an impeller member 22 driven by an electric motor 20. If it is necessary entirelyor partly to remove the digestion chemicals prior to the refining steps such removal is most suitably efliected by washing the stock in bins, on sieves or by means of presses located between the centricleaner 10 and the bin 18.

The pulp suspension is now conducted through a conduit 24 which may house a pump 26, to a first refiner 28 driven by an electric motor 30. In this refiner a grinding is effected at a temperature which must not surpass 60 C. As the substantial part of the energy supplied by the motor 30 is converted to heat, the temperature rise will thus be 20 C. This implies that the suspension must be cooled prior to, and/or under, the next grinding or refining step. In all refiners there may prevail a pressure of 1-3 kilograms per square centimeter for example. The refiners may be of the type shown and described in the US. patent specification No. 2,891,733.

As mentioned already, a temperature of 60 C. exists in the discharge conduit 32 from the refiner 28. A cooling is now effected according to the invention in an evaporator 34, in which a negative pressure is maintained which in the embodiment in consideration is an absolute pressure of 55 mm. Hg corresponding to a boiling point of the water at 40 C. Preferably, a throttling or pressure reducing valve 36 is provided in the conduit 36 and further the conduit 32 may be given a height assisting in keeping a higher pressure in the refiner 28 than in the evaporator 34. The suspension is introduced tangentially into said evaporator.

In the evaporator 34, so much water is evaporated that the temperature of the suspension is lowered to 40 C. The produced steam escapes through a conduit 38 to a condenser 40 fed with cooling water through a pipe 42. Said condenser 40 is through a conduit 44 in communication with a vacuum pump 46. The condensate is withdrawn from the condenser 40 through a pipe 48 which with its lower end is located below the liquid level in an open vessel 50 and which has a length resulting in a hydrostatic pressure preponderating over the negative pressure in the condsener 40.

The pulp suspension cooled in the evaporator 34 is fed through a conduit 52 and a pump 53 into a second refiner 54 driven by an electric motor 56. In this second refiner a renewed grinding is effected under rise of temperature so that the pulp suspension entering the discharge conduit 58 again has been heated to 60 C. Through a conduit 60 provided with a throttle valve 62 part of the ground pulp may be returned to the evaporator 34 for repeated cooling therein and a subsequent grinding treatment in the refiner 54.

The conduit 58 is through a throttle valve 64 connected to a second evaporator 66 in which a negative pressure is kept in the same manner as in the evaporator 34 to cause the pulp suspension to be cooled to 40 C. by evaporation of water. The developed vapors escape through a conduit 68 to a condenser 70 which through a conduit 72 is in connection with the vacuum pump 46 and which through the pipe 42 is supplied with cooling water. The condensate is discharged from the condenser 70 through the conduit 74 in which a liquid column is built up which is capable of surmounting the negative pressure in the condenser. The conduit 74 is extended downwardly below the liquid level in an open vessel 76 in the same manner as the conduit 48.

From the evaporator 66 a conduit 78 which may house a pump 79 extends to a third refiner 80 driven by an electric motor 82. In this refiner still another grinding of the pulp is effected which then leaves through a conduit 84 controlled by a valve 86 for a following treating step which does not form part of the present invention and therefor not described here. The temperature of the leaving pulp is this time also 60 C. Part of the fiber pulp may be recycled through a conduit 88 provided with a throttle valve 89 to the evaporator 66.

In order to avoid a cooking in the refiners between the grinding members thereof it is suitable to make use of the pumping efiect of the refiners and, as is easily understood from the explanation above, to raise the pressure in the grinding casing thereof either by a throttling of the discharge conduit and/ or by placing the evaporator on such level as to obtain a suitable hydrostatic counterpressure.

If the refining is conducted with recycling of cooled refined pulp it is possible to operate with somewhat higher pressure on/ and corresponding higher temperature of, the entering pulp suspension w'thout the temperature during the grinding process surpassing the allowable upper limit. The quantity of pulp passed through each refiner per time unit becomes greater and the suspension becomes capable of taking up more heat per each grade of temperature rise than in the case where no recycling is applied.

Provided that the refiner is constructed in a specific manner, it is also possible directly to produce negative pressure in their interior and continuously to remove the produced steam at the same rate as the forming thereof and in this manner to conduct the grinding operation at a constant temperature. I

FIGS. 2 and 3 illustrate diagrammatically a constructive embodiment of this modification of the method. A refiner 90 is provided with a horizontal stationary cylinder 91 within which a grinding member 92 is rotatably mounted. This member is formed with a central, four-edged boxlike beam which at its ends has axle journals 94, 96. The axle journal 94 is connected to a driving motor 98. The grinding member 92 has further four wings which during rotation of the member knead the material against the interior wall of the cylinder 91 which wall may be grooved to increase the friction. The beam-formed portion of the grinding member has perforations 102 and the axle journal 96 is provided with a central bore and by means of a box'device 104 and a stationary pipe conduit 106 connected to an injection condenser 108. This latter is fed with cooling water through a conduit 1110 and kept at negative pressure by means of a vacuum pump 112.

In operation of the refiner 90, a negative pressure corresponding to a boiling point of the water of 55 C., for example, is applied to the cylinder 91. A relatively concentrated pulp suspension the concentration of which may amount to between 10 and 20 percent, is fed into the cylinder from a container 114 through a conduit 116 in which is disposed a sluice or throttle valve 118. The ground pulp is discharged through a conduit 120. The pulp during its passage through the cylinder is ground by the grinding member 92, the developed heat being transformed to steam which is continuously withdrawn 5 through the bored journal 96 to the condenser 108. The heat escapes in vapor enthalpy and a constant temperature will prevail in the suspension.

The ground pulp is conveyed through the conduit 120 to a container 122 and Withdrawn for further treatment or working through a conduit 124 which is provided with a pump 126 and a valve 128. From the condenser 108 a conduit 130 extends to the container 122 for maintaining the desired negative pressure in said container. Through a conduit 132 housing a valve 134 pulp suspension may 10 be recycled by the pump 126 until the desired degree of .refining has been reached.

A conduit 136 extends from the condenser 108 down into a drain container 138, said conduit having a length surpassing the length corresponding to the negative pressure in the condenser.

Since the pulp concentration rises during the grinding operation through evaporation, water must be supplied continuously if the concentration shall be kept constant during the whole grinding process.

While two more or less specific embodiments of the invention have been shown and described, it is to be understood that this is for purpose of illustration only, and that the invention is not to be limited thereby, but its scope is to be determined by the appended claims.

What we claim is:

l. The method of continuously cooling pulp suspensions in water and maintaining, during the grinding operation, the temperature constant at an arbitrary level and within arbitrary limits within a temperature range of 20100 C. characterized in the pulp suspension being exposed to a pressure lower than atmospheric pressure to bring about a boil-01f of water and a cooling produced thereby down to the temperature to which the pulp suspension shall be cooled and at which it shall be kept cold.

2. The method of claim 1, characterized in that the predetermined quantity of cold refined pulp is recycled to the refiner.

3. The method of claim 1, characterized in that the pulp suspension is ground continuously in at least two steps of refiners coupled in series and that a cooling of refined pulp is etfected during the various steps.

4. The method according to claim 1, characterized in that the cold, refined pulp is recycled to at least one of the refiners coupled in series.

References Cited UNITED STATES PATENTS 962,118 6/1910 Bradley 241 2,689,688 9/ 1954 Ball 241-23 X 2,841,339 7/ 1958 Gilmore 241-65 X FOREIGN PATENTS 519,758 4/ 1940 Great Britain.

WILLIAM w. DYER, 111., Primary Examiner. HARRY F. PEPPER, JR., Examiner. 

1. THE METHOD OF CONTINUOUSLY COOLING PULP SUSPENSIONS IN WATER AND MAINTAINING, DURING THE GRINDING OPERATION, THE TEMPERATURE CONSTANT AT AN ARBITRARY LEVEL AND WITHIN ARBITRARY LIMITS WITHIN A TEMPERATURE RANGE OF 20*-100*C. CHARACTERIZED IN THE PULP SUSPENSION BEING EXPOSED TO A PRESSURE LOWER THAN ATMOSPHERIC PRESSURE TO BRING ABOUT A BOIL-OFF OF WATER AND A COOLING PRODUCED THEREBY DOWN TO THE TEMPERATURE TO WHICH THE PULP SUSPENSION SHALL BE COOLED AND AT WHICH IT SHALL BE KEPT COLD. 